How Is A Song By Someone If He/She Doesn't Even Sing In It?

I don't get it. I've run into this a couple of times. Like in In Need a Doctor by Dr. Dre featuring Skylar Grey and Eminem, Dr. Dre only has one verse. I sort of get that one 'cause he does sing in it, but in DJ Khaled's single, I'm On One, he only yells once, and the rest is done by Drake, Ross, and Lil Wayne. How does that work? Is is because he made the song and decided who sings in it or something like that? The same goes for the song, Until We Bleed. It's technically by Kleerup, but Lykke Li sings the whole thing.